In the prior art, it is known to use a single power supply to control the operation of a laser, such as a CO.sub.2 laser, in two modes: a continuous or CW mode, wherein power is supplied continuously to the laser, and a pulsed mode, wherein power is intermittently supplied to the laser, such that the laser discharge tube emits pulses of laser power.
Although the power supply is designed to operate in two modes, the electrical requirements of the two modes of operation are quite different. Thus, in the pulsed mode, momentary currents of several times the average current of the CW mode is desired. One prior art solution was to design the entire power supply to take on the higher current requirement of operation in the pulsed mode. This meant designing all of the components of the power supply to handle the large current requirement.
In an effort to reduce the number of components required to handle the large current, a prior art power supply used a resonant-mode solid-state power supply that consisted of two separately controlled stages with capacitive energy storage between the two stages. The first stage is termed the resonant converter. The capacitive energy storage between the stages is termed the D.C. Link. The second stage is termed the resonant inverter.
The converter receives high D.C. voltage, typically 600 volts D.C., from a rectifier. Sinusoidal current pulses are switched from the input D.C. voltage received, operating at a frequency which is controllable. The pulsed D.C. current is then rectified to produce a second D.C. voltage, which is supplied to the D.C. Link. The control of the frequency of the D.C. pulse operation controls the average output current of the converter. A higher frequency results in greater average current.
The inverter receives the D.C. voltage from the converter, during CW operation, or from the D.C. Link during pulsed operation, and converts the D.C. voltage into an A.C. voltage.
In the CW mode of operation, the converter supplies D.C. energy directly and continuously to the inverter across the DC link. In the pulsed mode of operation, the converter supplies energy to the D.C. Link, which stores the energy. The inverter then takes the energy from the D.C. Link intermittently, as needed. Since the inverter supplies energy to the laser in the intermittent or pulsed mode, the inverter needs to be designed for the high peak current of pulsed mode operation. However, if the average power used in the pulse mode does not exceed that of the power in the CW mode, the converter can be designed for the average power, not the peak power. The peak energy, required in the pulsed mode, is supplied from the DC Link, with the DC Link recharged by the converter between pulses. Thus, in certain circumstances, only the inverter portion of the power supply needs to be designed with the higher peak current operation of the pulsed mode.
The regulation or control of such a power supply consisted of monitoring the output current supplied to the laser discharge tube. Typically, this is done by using a resistor to generate a proportional voltage, and is compared to an analog current command. The analog current command signal is a signal generated by the user or by an CNC (Computer Numeric Control), or other means, to establish a set point at which the output current of the power supply is to be regulated. The frequency of the converter is modulated to maintain the output current constant under load and input variances. This works relatively well for the steady-state conditions of the CW mode.
In the pulse mode, however, the inverter is switched on and off thus opening and closing the current control loop. Between pulses, when the inverter is off, the converter will run at maximum frequency because there is no output current. The voltage on the DC link will rise until the converter "stalls" at the open circuit limit. The converter will "stall" when the voltage across it is approximately zero. This occurs when the DC Link voltage is approximately equal to the DC voltage, supplied as the input to the converter. The open circuit voltage is, therefore, purely dependent on the line voltage, which is unregulated.
Further, at the beginning of a pulse, when the inverter turns on, the peak current processed by the inverter is dependent upon the difference between the open circuit DC link voltage and the reflected voltage from the load. Since the open circuit D.C. Link voltage is unregulated, the peak current from the inverter is unregulated for line variations.
In addition, if the pulse current is higher than the current command, the converter cannot be modulated to achieve control because it can only raise the DC link voltage not lower it. The system runs unregulated until the inverter takes enough energy out of the DC link capacitance to bring the voltage into the regulation range. If the pulse ends before this occurs, then there is no regulation during the pulse.
To prevent the power supply from oscillating between full-off and full-on, and to achieve some control during the pulse mode, the control amplifier is typically bandwidth limited to achieve an averaging effect such that the average output current is controlled to meet the current command, if possible. This leaves the converter running at some average frequency during the entire pulse period. Because of this, if high peak current is needed, the average output current will be high, and if low average current is needed, then the initial peak is reduced excessively. In addition, because the bandwidth of the controller has been limited, the CW mode performance is also degraded severely.